Early Photocycle Intermediates K and L

In 1999 Landau and co-workers published a set of 2.1 A structures for a 35%-occupied, low-temperature K intermediate (Edman et al., 

1 A resolution, the two closely overlapping retinal configurations (all Irat/.s. 15 mil/ 

Only a high-resolution structural method that overcomes the difficulties of deconvoluting multiple, spatially highly overlapping structures (Fig. 4) is likely to be able to accurately answer this question. The spectral overlap problem is a principal one since even under ideal conditions it is not possible to achieve occupancies of the K or L intermediate higher than about 50% and 70%, respectively, because of the extensive spectral overlap between the ground state and both the K and the L intermediate (Amax differences of +22 nm and —28 nm, respectively) (Balashov and Ebrey, 2001). 

Landau and co-workers reported a merohedrally twinned 2.1 A structure, claimed to be of a 70%-occupied, low-temperature L intermediate (Royant et al., 2000). The claimed L-intermediate occupancy was later corrected to 36%, in a mixture with 40% ground state, 12% K and 12% M intermediate (Royant et al., 2001b). The main interpretation of this structure was that 

Asp 85, to approach closer to the positively charged primary proton donor, the Schiff base. The primary proton transfer event would then neutralize these two groups, canceling their electrostatic attraction and facilitating a relaxation of helix C to a less strained geometry. 

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